The present invention relates to cushioning devices for footwear and, more particularly, to a gas-filled cushioning device which includes an elastomeric barrier material for selectively controlling the diffusion of inert gases while allowing controlled diffusion of gases normally contained in the atmosphere, with the cushioning device being particularly employed in footwear products.
Shoes, and particularly athletic shoes, can be described as including two major categories of components namely, a shoe upper and a sole. The general purpose of the shoe upper is to snugly and comfortably enclose the foot. Ideally, the shoe upper should be made from an attractive, highly durable, yet comfortable material or combination of materials. The sole, which also can be made from one or more durable materials, is primarily designed to provide traction, and to protect the wearer""s feet and body during any use consistent with the design of the shoe. The considerable forces generated during uses such as athletic activities require that the sole of an athletic shoe provide enhanced protection and shock absorption for the feet, ankles and legs of the wearer. For example, impacts which occur during running activities can generate forces of up to two to three times body weight; certain other activities, e.g., playing basketball, have been known to generate forces of up to approximately 6-10 times an individual""s body weight. Accordingly, many shoes and, more particularly, many athletic shoe soles are now provided with some type of resilient, shock-absorbent material or shock-absorbent components to cushion the user during strenuous athletic activity. Such resilient, shock-absorbent materials or components have now commonly come to be referred to in the shoe manufacturing industry as the mid-sole.
More specifically, it has been a focus of the industry to seek a mid-sole design which achieves an effective impact response in which both adequate shock absorption and resiliency are appropriately taken into account. Such resilient, shock-absorbent materials or components could also be applied to the insole portion of the shoe, which is generally defined as the portion of the shoe upper directly underlining the plantar surface of the foot.
A specific focus in the shoe manufacturing industry has been to seek mid-sole or insert structure designs which are adapted to contain fluids, in either the liquid or gaseous state, or both. Examples of gas-filled structures which are utilized within the soles of shoes are shown in U.S. Pat. No. 900,867 entitled xe2x80x9cCushion for Footwearxe2x80x9d which issued Oct. 13, 1908, to Miller; U.S. Pat. No. 1,069,001 entitled xe2x80x9cCushioned Sole and Heel for Shoesxe2x80x9d which issued Jul. 29, 1913, to Guy; U.S. Pat. No. 1,304,915 entitled xe2x80x9cPneumatic Insolexe2x80x9d which issued May 27, 1919, to Spinney; U.S. Pat. No. 1,514,468 entitled xe2x80x9cArch Cushionxe2x80x9d which issued Nov. 4, 1924, to Schopf; U.S. Pat. No. 2,080,469 entitled xe2x80x9cPneumatic Foot Supportxe2x80x9d which issued May 18, 1937, to Gilbert; U.S. Pat. No. 2,645,865 entitled xe2x80x9cCushioning Insole for Shoesxe2x80x9d which issued Jul. 21, 1953, to Towne; U.S. Pat. No. 2,677,906 entitled xe2x80x9cCushioned Inner Sole for Shoes and Method of Making the Samexe2x80x9d which issued May 11, 1954, to Reed; U.S. Pat. No. 4,183,156 entitled xe2x80x9cInsole Construction for Articles of Footwearxe2x80x9d which issued Jan. 15, 1980, to Rudy; U.S. Pat. No. 4,219,945 entitled xe2x80x9cFootwearxe2x80x9d which issued Sept. 2, 1980, also to Rudy; U.S. Pat. No. 4,722,131 entitled xe2x80x9cAir Cushion Shoe Solexe2x80x9d which issued Feb. 2, 1988, to Huang; and U.S. Pat. No. 4,864,738 entitled xe2x80x9cSole Construction for Footwearxe2x80x9d which issued Sep. 12, 1989, to Horovitz; all of which are incorporated herein by reference. As will be recognized by those skilled in the art, such gas filled structures (often referred to in the shoe manufacturing industry as xe2x80x9cbladdersxe2x80x9d) typically fall into two broad categories, namely (1) xe2x80x9cpermanentlyxe2x80x9d inflated systems such as those disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 and (2) pump and valve adjustable systems as exemplified by U.S. Pat. No. 4,722,131. By way of further example, athletic shoes of the type disclosed in U.S. Pat. No. 4,182,156 which include xe2x80x9cpermanentlyxe2x80x9d inflated bladders have been successfully sold under the trade mark xe2x80x9cAir Solexe2x80x9d and other trademarks by Nike, Inc. of Beaverton, Oregon. To date, millions of pairs of athletic shoes of this type have been sold in the United States and throughout the world.
The permanently inflated bladders are typically constructed under methods using a flexible thermoplastic material which is inflated with a large molecule, low solubility coefficient gas otherwise referred to in the industry as a xe2x80x9csuper gas,xe2x80x9d such as SF6. By way of example, U.S. Pat. No. 4,340,626 entitled xe2x80x9cDiffusion Pumping Apparatus Self-Inflating Devicexe2x80x9d which issued Jul. 20, 1982, to Rudy, which is expressly incorporated herein by reference, discloses a pair of elastomeric, selectively permeable sheets of film which are formed into a bladder and thereafter inflated with a gas or mixture of gases to a prescribed pressure which preferably is above atmospheric pressure. Ideally, the gas or gases utilized have a relatively low diffusion rate through the selectively permeable bladder to the exterior environment while gases such as nitrogen, oxygen and argon (which are contained in the atmosphere and have a relatively high diffusion rate) are able to penetrate the bladder. This produces an increase in the total pressure within the bladder by the additive nature of the partial pressures of the nitrogen, oxygen and argon which diffuse into the bladder from the atmosphere and the partial pressures of the gas or gases contained initially injected into the bladder upon inflation. This concept of an almost total xe2x80x9cone-wayxe2x80x9d addition of gases to enhance the total pressure of the bladder is now known in the art as xe2x80x9cdiffusion pumping.xe2x80x9d
In a diffusion pumping system, there is a period of time involved before a steady state of internal pressure is achieved. The period of time is related to the bladder material used and the choice of gas or gases contained in the bladder. For example, oxygen tends to diffuse into the bladder rather quickly with the effect being an increase in the internal pressure of approximately 2.5 psi. In contrast, over the course of a number of weeks nitrogen gas will gradually diffuse into the bladder resulting in an increase of pressure to approximately 12.0 psi. This gradual increase in bladder pressure typically causes an increase in tension in the skin, resulting in a volume increase due to stretching. This effect is commonly referred to in the industry as xe2x80x9ctensile relaxationxe2x80x9d or xe2x80x9ccreep.xe2x80x9d Thus, the initial selection of materials employed in the bladder and the choice of the captive gas or gas mixture utilized to initially inflate the bladder is critical to achieving a bladder which is essentially permanently inflated at a desired internal pressure and which therefore maintains a desired internal pressure over an extended period of time.
Prior to and shortly after the introduction of the Air Sole(trademark) athletic shoes, many of the mid-sole bladders employed in the industry consisted of a single layer gas barrier type film made from polyvinylidene chloride based materials such as Saran(copyright) (which is a registered trademark of the Dow Chemical Co.). These materials which, by their nature are rigid plastics, are less than ideal from the standpoint of flex fatigue, heat sealability, elasticity, and degradation. Attempts to address these limitations by creating bladder films made by techniques such as laminations and coatings (which involve one or more barrier materials in combination with a flexible bladder material such as various thermoplastics) then present a wide variety of problems typical of such combinations. Such difficulties with composite constructions typically include layer separation; peeling; gas diffusion or capillary action at weld interfaces; low elongation which leads to wrinkling of the inflated product; cloudy appearing finished bladders; reduced puncture resistance and tear strength; resistance to formation via blow-molding and/or heat-sealing and/or R-F welding; high cost processing; and difficulty with foam encapsulation and adhesive bonding; among others.
The art has attempted to address these problems (created by trying to laminate two or more dissimilar materials to balance the advantages and disadvantages of any single material) by the use of tie-layers or adhesives between the layers in preparing laminates. The use of such tie layers or adhesives help solve some of the difficulties noted above but generally prevent regrinding and recycling of any waste materials created during product formation back into an usable product, and thus, also contribute to high cost of manufacturing and relative waste. These and other short comings of the prior art are described in more extensive detail in U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176 which are hereby expressly incorporated by reference.
With the extensive commercial success of the products such as the Air Sole(trademark) shoes, consumers have been able to enjoy a product with a long service life, superior shock absorbency and resiliency, reasonable cost, and inflation pressure stability, without having to resort to pumps and valves. Thus, in light of the significant commercial acceptance and success that has been achieved through the use of long life inflated gas filled bladders, it is highly desirable to develop advancements to solve the few remaining disadvantages associated with such products. The goal then is to provide flexible, xe2x80x9cpermanentlyxe2x80x9d inflated, gas-filled shoe cushioning components which meet, and hopefully exceed, performance achieved by such products as the Air Sole(trademark) athletic shoes offered by Nike, Inc.
One key area of potential advancements stems from a recognition that it would be desirable to employ xe2x80x9ccapturexe2x80x9d or xe2x80x9ccaptivexe2x80x9d gases other than the large molecule, low solubility coefficient xe2x80x9csuper gasesxe2x80x9d as described in the ""156, ""945 and ""738 patents, replacing them with less costly and possibly more environmentally friendly gases. For example, U.S. Pat. Nos. 4,936,029 and 5,042,176 specifically discuss the methods of producing a flexible bladder film that essentially maintains permanent inflation through the use of nitrogen as the captive gas. As further described in U.S. Pat. No. 4,906,502, also expressly incorporated herein by reference, many of the perceived problems discussed in the ""029 and ""176 patents are solved by the incorporation of mechanical barriers of crystalline material into the flexible film such as fabrics, filaments, scrims and meshes. Again, significant commercial success for footwear products using the technology described in ""502 patent (marketed under the trademark Tensile Air(trademark) by Nike, Inc.) has been achieved. The bladders utilized therein are typically comprised of a thermoplastic urethane laminated to a core fabric three-dimensional, double bar Raschel knit nylon fabric, having SF6 as the captive gas contained therein. As discussed in the ""502 patent, such bladder constructions have reduced permeability to SF6, nitrogen and other captive gases.
Exemplary of an accepted method of measuring the relative permeance, permeability and diffusion of different film materials is set forth in the procedure designated as ASTM 1434V. According to ASTM 1434V, permeance, permeability and diffusion are measured by the following formulas:   Permeance                    (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                                        Permeance                                                                              (                GTR                )                            /                              (                                  press                  .                                      xe2x80x83                                    ⁢                  diff                  .                                )                                                        =                        cc          .                                      (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                Permeability                              (                      quantity            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            gas                    )                xc3x97                  (                      film            ⁢                          xe2x80x83                        ⁢            thick                    )                                      (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                                        Permeability                                                                              (                GTR                )                            xc3x97                                                (                                      film                    ⁢                                          xe2x80x83                                        ⁢                    thick                                    )                                /                                  (                                      press                    .                                          xe2x80x83                                        ⁢                    diff                    .                                    )                                                                        =                        cc          .                      xe2x80x83                    ⁢                      (            mil            )                                                (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                Diffusion                    (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                      =                                                      Gas              ⁢                              xe2x80x83                            ⁢              Transmission              ⁢                              xe2x80x83                            ⁢              Rate                                                                          (              GTR              )                                          =                        cc          .                                      (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                              
By utilizing the above listed formulae, the gas transmission rate in combination with a constant pressure differential and the film""s thickness, can be utilized to define the movement of gas under specific conditions. In this regard, the preferred gas transmission rate (GTR) for a bladder in an athletic shoe component which seeks to meet the rigorous demands of fatigue resistance imposed by heavy and repeated impacts has a gas transmission rate (GTR) value of less than about 10, more preferably less than about 7.5, still more preferably less than about 5., and still more preferably, a (GTR) value of 2.0 or lower, preferably as measured by the above procedure.
In addition to the aforementioned, the ""029 and ""176 patents also discuss problems encountered with previous attempts to use co-laminated combinations of plastic materials at least one of which is selected to operate as a barrier to oxygen. In this regard, the principal concern was the lack of fatigue resistance of the barrier layer. As described in the ""176 patent, a satisfactory co-lamination of polyvinylidene chloride (such as Saran(copyright)) and a urethane elastomer would require an intermediate bonding agent. Under such a construction, relatively complicated and expensive processing controls would be required, such as strict time-temperature relationships and the use of heated platens and pressures, coupled with a cold press to freeze the materials together under pressure. Additionally, using adhesive tie layers or incorporating crystalline components into the flexible film at high enough levels to accomplish a gas transmission rate of 10 or less, dramatically reduces the flexibility of the film.
It is therefore, a principal object of the present invention to provide an inflatable cushioning device that is essentially permanently inflated with nitrogen or another environmentally desirable gas or combination of gases which meet the goals of flexibility, durability and low cost.
It is another object of the present invention to provide a cushioning device having a permeable barrier material with a gas transmission rate value of 10.0 or less.
It is still another object of the present invention to provide a cushioning device which substantially resists peeling between layers, is relatively transparent and economical to manufacture.
It is yet another object of the present invention to provide a cushioning device where the barrier layer substantially resists delamination and does not require a tie layer between the barrier layer and the flexible layers.
It is a further object of the present invention to provide a cushioning device which is formable utilizing the various techniques including, but not limited to, blow-molding, tubing, sheet extrusion, vacuum-forming, heat-sealing and RF welding.
It is an additional object of the present invention to provide a gas cushioning device which prevents gas from escaping along interfaces between the materials via capillary action.
It is yet another object of the present invention to provide a cushioning device which allows for normal footwear processing such as encapsulating the cushioning device in formable material.
The above list is not intended to be exhaustive of the objects or advantages of the present invention.
To achieve the foregoing and other objectives, the present invention provides a cushioning device which features one or more novel gas-filled membranes with both the desired physical properties of a thermoplastic elastomeric film and with improved barrier properties for retaining nitrogen gas and other captive gases. The gas-filled membranes are formulated so as to selectively control the rate of outward diffusion of certain captive gases (e.g., nitrogen and super gases) through the membranes as well as enable diffusion pumping of externally mobile gases (e.g., argon, oxygen, carbon dioxide and the other gases which are present in ambient air), into the gas-filled membranes.
The gas-filled membranes of the present invention are preferably constructed of at least two flexible materials which together act as a barrier, and which are preferably elastomeric and polar in nature and capable of forming products in a variety of geometries. Ideally, the flexible barrier materials utilized in accordance with the teachings of the present invention should contain the captive gas within an interior compartment of the gas-filled membrane for a relatively long period of time. In a highly preferred embodiment, for example, the gas-filled membrane should not lose more than about 20% of the initial inflated gas pressure over a period of two years. In other words, products inflated initially to a steady state pressure of between 20.0 to 22.0 psi should retain pressure in the range of about 16.0 to 18.0 psi after a period of about two years.
Additionally, the barrier materials utilized should be flexible, relatively soft and compliant and should be highly resistant to fatigue and be capable of being welded to form effective seals typically achieved by RF welding or heat sealing. The barrier material should also have the ability to withstand high cycle loads without failure, especially when the barrier material utilized has a thickness of between about 5 mils to about 50 mils. Another important characteristic of the membranes of the present invention is that they should be processable into various shapes by techniques used in high volume production. Among these desirable techniques known in the art are blow molding, injection molding, vacuum molding, rotary molding, transfer molding and pressure forming. The membranes of the present invention should be preferably formable by extrusion techniques, such as tubing or sheet extrusion, including extrusion blow molding particularly at sufficiently high temperatures to attain the desired xe2x80x9cadhesivexe2x80x9d or xe2x80x9cchemicalxe2x80x9d bonding as will be described in greater detail below without the use of a separate adhesive or tie-layer. These aforementioned processes should give rise to products whose cross-sectional dimensions can be varied.
As mentioned above, a significant feature of the membranes of the present invention is the controlled diffusion of mobile gases through the barrier layer and retention of captive gases therein. By the present invention, not only are the super gases usable as captive gases, but nitrogen gas may also be used as a captive gas due to the improved performance of the barrier. The primary mobile gas is oxygen, which diffuses relatively quickly through the barrier, and the other mobile gases may be any of the gases normally present in air except nitrogen. The practical effect of providing a barrier material for which nitrogen gas is suitable as a captive gas is significant.
For example, the membrane may be initially inflated with nitrogen gas or a mixture of nitrogen gas and one or more super gases or with air. If filled with nitrogen or a mixture of nitrogen and one or more super gases, an increment of pressure increase results from the relatively rapid diffusion of oxygen gas into the membrane, since the captive gas is essentially retained within the membrane. This effectively amounts to an increase in pressure of not greater than about 2.5 psi over the initial inflation pressure and results in a relatively modest volume growth of the membrane of between 1 to 5%, depending on the initial pressure. However, if air is used as the inflatant gas, oxygen tends to diffuse out of the membrane while the nitrogen is retained as the captive gas. In this instance, the diffusion of oxygen out of the membrane and the retention of the captive gas results in an incremental decrease of the steady state pressure over the initial inflation pressure.
This invention has many other advantages which will be more apparent to the skilled artisan from consideration of the various forms and embodiments of the present invention. Such embodiments are shown in the accompanying drawings and form a part of the present specification. The various embodiments will now be described in greater detail for the purpose of illustrating the general principles of the invention without considering the following detailed description in the limiting sense.